A brushing hydrophobic agent of gypsum was prepared with organic silicon monomer (ASID), actyflon-G02, actyflon-G04 and (meth)acrylate monomer by solution polymerization method. The influence factors including the reaction temperature and the introducing model of monomers were investigated. Effect of the hydrophobic agent on water absorption of gypsum samples was also studied. The experimental ... [Show full abstract] results indicated that the early water resistance of gypsum samples with the agent was obviously improved. However the water absorption became large when gypsum samples were used for a long period of time, and it is applied to parts of the building that short-term subjected to moisture intrusion.